One type of a relay valve apparatus well known in the prior art is described in a brochure published by the Graubremse Company, "Graubremse Ausgabe IAA 1987", Page 8.
The disclosed relay valve apparatus, which is described in this brochure, includes two relay valves located in a housing with their longitudinal axes parallel to one another so that the fluid pressure inlet chambers of the relay valve are connected by means of a common pressure connection with a fluid pressure source. In the same housing, there are additional valve mechanisms to influence the control pressure.
The prior art relay valve apparatus requires a special and relatively complex housing structure which is expensive to manufacture and difficult to maintain.